1. Field of the Invention
The present invention relates to a liquid discharging head used as, for example, a printer head of an inkjet printer. More particularly, the present invention relates to a technology for restricting deformation of a nozzle member caused by the discharge of a liquid.
2. Description of the Related Art
A printer head of an inkjet printer is known as a related liquid discharging head of a liquid discharging device. FIG. 11 is an exploded perspective view of a thermal printer head (hereafter simply referred to as “head”) 1.
In FIG. 11, heating elements (such as heating resistors) 13 are disposed on the top surface of a semiconductor substrate 15 of the head 1. A barrier layer 16 defining ink chambers 12 is disposed on the semiconductor substrate 15. A nozzle sheet 17 having a plurality of nozzles 18 (that is, through holes that are substantially trapezoidal in cross section along center axial lines) is disposed on the barrier layer 16. The nozzles 18 and the heating elements 13 are disposed so that the center axial lines of the nozzles 18 pass through the centers of the heating elements 13 disposed under the nozzles 18.
The ink chambers 12 are formed by the semiconductor substrate 15 having the heating elements 13 disposed thereon, the barrier layer 16, and the nozzle sheet 17 having the nozzles 18.
In the specification, a portion formed by one ink chamber 12, the heating element 13 disposed in the one ink chamber 12, and the nozzle sheet 17 having the nozzles 18 and disposed above the heating element 13 is called a liquid discharging unit. In other words, the head 1 comprises a plurality of liquid discharging units disposed in parallel. (The same applies to a head 11 of an embodiment described later.)
In FIG. 11, the center of each nozzle 18 is disposed in a straight line in the direction of arrangement of the nozzles 18. Therefore, the center of each heating element 13 is also disposed in a straight line. The nozzles 18 (and the heating elements 13) are disposed in a straight line because, from the viewpoint of a nozzle 18 production technology, they are not particularly difficult to dispose in a straight line. Similarly, the heating elements 13 disposed right below the nozzle sheet 17 are disposed in a line straight line because it is easier to disposed them in a straight line.
A method in which the nozzles 18 are intentionally not disposed in a straight line is also known (refer to U.S. Pat. No. 4,812,859).
FIGS. 12A and 12B are plan views of a row of nozzles 18 and rows of nozzles 18 and dots formed by the row of nozzles 18 and the rows of nozzles 18, respectively. In the figures, the upper side shows the arrangement of the nozzles 18, and the lower side shows the arrangement of the formed dots.
In FIG. 12A, nozzles A1 to A4 and nozzles B1 to B4 are disposed in a straight line as in FIG. 11. In contrast, FIG. 12B, the nozzles A1 to A4 and B1 to B4 are not disposed in a straight line as disclosed in U.S. Pat. No. 4,812,859.
In FIG. 12, four nozzles 18 are defined as one block. The number of nozzles 18 to be defined as one block depends upon, for example, the refill property of ink (that is, the refilling performance for ink lost due to discharge with respect to time), heating, head life, and the degree of liquid surface (meniscus) interference caused by the discharge. Ordinarily, 16, 32, or 64 nozzles are defined as one block. Here, for convenience of explanation, four nozzles 18 are defined as one block.
Ordinarily, when a plurality of nozzles 18 are disposed in one row in a thermal printer head, ink droplets are not discharged from all of the nozzles 18 at the same time or from adjacent nozzles 18 at the same time. The first reason for not carrying out such discharging operations is to eliminate power consumption problems and heating problems arising from the power consumption problems.
The second reason is that, since a common flow path for supplying ink to all of the ink chambers 12 is disposed close to the nozzles 18, when ink droplets are discharged from adjacent nozzles 18 at the same time, interference (crosstalk) is increased, thereby preventing the discharged ink amount from being easily stabilized, and causing considerably variations in the discharge directions of the ink droplets. Therefore, ordinarily, the following method is used. A predetermined number of nozzles 18 is defined as one group, and only one nozzle 18 is allowed to discharge ink in one group at all times. Each group is concurrently operated so that nozzles 18 that discharge ink droplets at the same time are always separated by a distance corresponding to the number of nozzles 18 in each group.
In FIG. 12A, the nozzle A1 of group A (consisting of the nozzles A1 to A4) and the nozzle B1 of group B (consisting of the nozzles B1 to B4) discharge ink droplets at the same time. Therefore, a dot formed by the nozzle A1 and a dot formed by the nozzle B1 are disposed horizontally in a straight line.
After the passage of a predetermined amount of time from the discharge, the nozzle A2 of the group A and the nozzle B2 of the group B discharge ink droplets at the same time. At this time, due to the time difference, a recording medium moves relative to the head during a time equivalent to the time difference, as a result of which dots are formed at slightly displaced locations from the previously formed dots. When a discharge command is subsequently similarly generated, dots are gradually formed downwards and rightwards in FIG. 12A.
In contrast, in FIG. 12B, since the positions of the nozzles 18 are displaced in a direction opposite to the direction of the formation of dots from the beginning by an amount corresponding to the aforementioned time difference, dots are formed in a straight line. In FIG. 12B, the amount of positional displacement caused by the movement of the recording medium relative to the head due to the time difference and the amount of positional displacement of the previously displaced nozzles 18 are set equal to each other.
Accordingly, a method for forming dots in a straight line without disposing the nozzles 18 in a straight line is known.
The nozzle sheet 17 is generally formed of a metallic foil or a thin polymeric material. It is very thin, that is, 10 to 30 μm, when used in, for example, recent high-resolution inkjet printers.
However, when an attempt is made to reduce the thickness of the nozzle sheet 17, the following problems arise.
FIG. 13 is a sectional view of a liquid discharging unit of an inkjet printer when it is designed on the assumption that an ink droplet of 4.5 picoliters is discharged at a nozzle pitch at 600 DPI. FIG. 13 corresponds to a sectional diagram of the head 1 of FIG. 11 along the central axial line of the nozzle 18 at a line connecting the centers of the nozzles 18.
The structure shown in FIG. 13 is formed on the semiconductor substrate 15 by either one of the following known technological methods. They are:
(1) A method for forming a circuit including the heating elements 13 on the semiconductor substrate 15 formed of, for example, silicon by a photomechanical technology, and adding the barrier layer 16 and the nozzle sheet 17 by a separate post-processing step, and
(2) A method for forming the structure as well as the nozzle sheet 17 on the semiconductor substrate 15 formed of, for example, silicon by the photomechanical technology.
Method (1) has the advantage that the material and processing method may be selected from a larger number of choices. However, it has the disadvantage that its manufacturing precision is less than that of method (2), which is a combination processing method, because the error in the postprocessing step and the error in the semiconductor processing step (pre-processing step) are generally different.
Although both these methods may be used to form practical liquid discharging units, the discharge performance and production costs of the liquid discharging units differ depending upon the dimensions of each part.
For example, in method (1), when the nozzle sheet 17 is formed by an electroforming process (which is a process which is the reverse of an electrolytic process) using nickel material, the thickness of the nozzle sheet 17 is proportional to, for example, the concentration of the electrolyte and the quantity of electricity. Therefore, the thicker the nozzle sheet 17, the longer the time required to carry out method (1) and the larger the amount of nickel used in the method. Consequently, costs are increased.
The inventor et al. have already proposed a technology for providing high-quality printing by reducing variations in the landing positions of ink droplets as a result of varying the direction of discharge of the ink droplets from the nozzles on the basis of, for example, earlier filed and undisclosed technologies in Japanese Patent Application Nos. 2003-037343, 2002-360408, and 2003-55236. When this technology is used, the thinner the nozzle sheet 17, the larger the amount of deflection of the ink droplets (refer to Japanese Patent Application No. 2003-351550).
In a liquid discharging head typically used in, for example, an inkjet printer, a nozzle sheet 17 having a relatively large thickness value of 20 μm to 30 μm is not rare. However, it may be necessary to achieve required performances using a thin nozzle sheet such as the nozzle sheet 17 shown in FIG. 13 depending upon the purpose of use.
Since the nozzle sheet 17 is always in contact with a liquid (ink), its liquid contact property with respect to the liquid (primarily referring to changes in the physical properties of the surfaces of the nozzles 18 and the melting of the nozzle sheet 17 due to its reaction with the liquid) needs to be considered. Therefore, the composition of the liquid may limit the materials which may be used for the nozzle sheet 17.
Due to the above-described circumstances, since the mechanical strength (Young's modulus, fatigue characteristics with respect to bending, etc.) of materials is limited, methods (1) and (2) give rise to problems in that, when the nozzle sheet 17 is thin, the discharge performance is impaired as a result of changes in pressure applied to the ink chambers 12 when discharging liquid droplets and in that the life is reduced as a result of, for example, repeated fatigue. Therefore, the thickness of the nozzle sheet 17 cannot be made equal to or less than a predetermined thickness value.
In other words, if the nozzle sheet 17 is a rigid body, and pressure is applied thereto by the discharging operation, the amount of deformation of the nozzle sheet 17 can be considered as being so small as to be negligible. Actually, however, the nozzle sheet 17 is deformed because a very high pressure is produced during the discharge.
FIG. 14 shows a photograph of the moment an ink droplet is actually discharged. The nozzle sheet 17 shown in FIG. 14 is formed by electroforming using nickel.
As shown in FIG. 14, the ink droplet is considerably elongated when it is actually discharged. Although the ink droplet is actually discharged downward, it is shown as being discharged upward in FIG. 14. It is observed that areas near the nozzles 18 of the nozzle sheet 17 are flexed when the discharging operation is carried out as shown in FIG. 14. (In FIG. 14, the nozzle sheet 17 is shown as being bulging upward.)
An ordinary discharge of a liquid droplet produces a relatively fine circular dot and satellites (small liquid droplets that fly off by the discharge of the main liquid droplet). As shown in FIG. 14, however, if the liquid droplet is discharged when the nozzle sheet 17 is flexed, a large satellite and a liquid droplet that is not circular are produced. Therefore, dots are often not aligned. FIG. 15 shows in enlarged form a photograph of the arrangement of dots formed when the nozzle sheet 17 is flexed as shown in FIG. 14. In FIG. 15, the pitch between the nozzles 18 (or dots) is represented by P.
As can be understood from the foregoing description, when the nozzle sheet 17 becomes thin, pressure changes during the discharge of liquid droplets cause the areas surrounding the nozzles 18 to flex. Therefore, a stable and a high-quality liquid discharge operation may not be carried out.